1. Field of the Invention
The present invention relates to thin film transistors and organic light emitting displays including the same. More particularly, the invention relates to a thin film transistor including a P-type semiconductor layer and an organic light emitting display including the same.
2. Description of Related Art
In general, semiconductor layers using amorphous silicon or poly silicon have been widely used in thin film transistors for organic light emitting displays. However, since semiconductor layers formed of amorphous silicon have lower mobility, it is difficult to use such a semiconductor layer as a driving circuit of a display panel requiring high operation speed. In contrast, poly silicon has higher mobility. However, the threshold voltage is non-uniform, generally requiring an additional compensating circuit. Furthermore, thin film transistors having semiconductor layers formed of amorphous silicon or poly silicon generate leakage currents from irradiation of light, which leads to a deterioration in the characteristics of the thin film transistors.
Recently, an oxide semiconductor has been proposed. For example, a thin film transistor using ZnO or an oxide semiconductor with the ZnO as a semiconductor layer has been proposed.
FIG. 1 is a cross-sectional view showing a conventional thin film transistor having a semiconductor layer formed of ZnO. With reference to FIG. 1, the thin film transistor 100 includes a source electrode 120a, a drain electrode 120b, a semiconductor layer 130, a gate insulating layer 140, and a gate electrode 150. The source electrode 120a and the drain electrode 120b are formed on an insulation substrate 110. The semiconductor layer 130 contacts the source and drain electrodes 120a and 120b, and is formed of ZnO. The gate insulating layer 140 and the gate electrode 150 are stacked on the semiconductor layer 130.
Since the ZnO semiconductor (or oxide semiconductor having ZnO) has a band gap of 3.4 (greater than that of light energy in a visible ray region), the leakage current of the thin film transistor is not increased due to the absorption of visible rays. However, the ZnO semiconductor (or oxide semiconductor having ZnO) is indicative of an N-type semiconductor layer due to oxygen vacancy, zinc interstitial, and hydrogen incorporation. In contrast, organic light emitting displays generally use P-type semiconductor layers.
When forming an organic light emitting display using a N-type semiconductor layer, a method of preparing the organic light emitting display using an inverted type organic light emitting diode has been proposed to address variations in data voltage due to the degradation of the organic light emitting diode. Here, the inverted type organic light emitting diode is an organic light emitting diode in which a cathode electrode, an emission layer, and an anode electrode are sequentially formed on a thin film transistor on a substrate.
However, in organic light emitting displays using inverted type organic light emitting diodes, the contact characteristics of the cathode electrode and the emission layer may be deteriorated, and defects in the emission layer due to the anode electrode being formed on the emission layer may occur. That is, the contact characteristics of the cathode electrode (formed of a silver (Ag) alloy) and the emission layer (formed of inorganic materials) may be deteriorated. Furthermore, when the anode electrode, such as indium tin oxide (ITO) or indium zinc oxide (IZO), is formed on the emission layer by sputtering, the emission layer can be damaged.